1. Field of the Invention
The invention relates in general to an optical rotary encoder and more specifically to a miniaturized optical encoder having an in situ laser beam generator, a novel heat dissipation system and a wave plate phase delay means.
2. Description of the Related Art
For some time now, optic encoder means have been used with great success for precise positioning applications. During this time it has been widely recognized by those skilled in the art that smaller is better. A smaller position tracking mechanism has obvious packaging and operational advantages such as for instance a smaller platform, increased beam intensity for diverging beam applications such as in the present invention, more responsive positioning system due to momentum reduction, reduced motor size requirements, etc. Limiting factors for farther size reduction are the physical size of the laser diode and photodetector packages, heat dissipation and optical component orientation and adjustment requirements.
Temperature parameter compensation is addressed in: U.S. Pat. No. 5,652,426 which is directed to an optical encoder that obtains displacement information by means of a twice-diffracted beam and a twice-transmitted beam to thereby compensate for changes of wavelength of a light beam due to temperature change.
Wave plate compensation is addressed in: U.S. Pat. No. 5,596,403 which is directed to an angular position measuring system having a source assembly including a laser driver, a conventional linear polarizer and a rotating half-wave plate. U.S. Pat. No. 5,677,768 discloses an encoder having a stationary and a rotating quarter-wave plate. A signal processor (for removing an unwanted component due to the rotating wave plate introducing a frequency shift proportional to the angular rotation) is mentioned but not described.
None of the above references addresses the temperature dissipation or wave plate orientation problems encountered when size reduction of optical encoders of the related art is attempted. Nor do the references address the problem that the physical size limitations of the laser diode and photo detector packages hampers further size reduction of optical encoder apparatus.
Accordingly it would be desirable to have a miniature optical encoder apparatus that is not limited by the size of the laser diode and photodetector packages. It would further be desirable if the miniature optical encoder apparatus included heat dissipation means for removing and dissipating heat from the entire apparatus. It would be further desirable if the miniature optical encoder apparatus included a combination of a wave plate phase delay means and signal processing means to provide for phase delay compensation/adjustment.
As has been mentioned earlier, the focus of this invention is to optimize and miniaturize the size of optical encoder apparatus. One of the necessary parameters that has to be addressed to miniaturize an optical encoder is of course heat control including most importantly heat dissipation. The invention recognizes that heat control plays an equally important role in optical encoders than as just a parameter to be controlled or optimized for miniaturization. For instance, uneven heating (including that due to uneven heat dissipation) anywhere in an optical encoder and especially in the base of the optical encoder can lead to degeneration of the optics due to uneven expansion or contraction of one or more elements and especially the base upon which the elements are mounted in the optical encoder apparatus, and degeneration of the epoxy attachment means for all of the encoder elements. In encoders of the Relevant Art, the heat generated by the laser diode is addressed because of course excess heat at the laser diode can interfere with its performance. For this reason, heat control mitigating elements are employed in typical laser diode packages. However, the present invention recognizes that heat comes from other sources besides the laser diode. Another heat source that has not been addressed by the Relevant Art is the heat that stems from the electric motor and drive shaft for movement of the rotary grating or other tracking means the optical encoder employs. Another source of heat is the ambient temperature of the environment in which the encoder is employed. Another source of heat are the electronic circuits that are both integral with and mounted on the optical encoder.
Accordingly, it would be desirable to have heat control apparatus that controls the heat profiles of the entire optical encoder architecture so that the laser diode performance is not affected, and that misalignment of the optics of the encoder will not result from heat induced material distortion. It would further be particularly desirable if the base mounting plate of the optical encoder were to be heat controlled so as to be operated in a heat range wherein laser diode output noise sources will be minimized.
Briefly the present invention recognizes that the limiting structures to size reduction for an optical encoder are the laser diode/photo diode package and the photodetector with amplifier packages. The invention includes removing the discrete components from the laser diode cover package and mounting the much smaller discrete components on a base framework. The photo detector with amplifier packages of the invention have an altered photodetector/amplifier geometry including reducing the length of the connection wire. This resulted in significant size reduction of the encoder, a reduction in required gain and resulting increase in bandwidth. The entire base framework is temperature cooled by disposing two thermoelectric coolers between the base framework and an adjacent heat sink. Such positioning of the two coolers between the base framework and it""s adjacent heat sink maintains the base framework at a controlled temperature regimen so as to stabilize all elements(both optical and electronic) mounted on the base framework, most importantly stabilizing both the laser diode""s and the photodetecto""s outputs. In addition this novel design stabilizes the entire footprint/base of the encoder from thermal effects, whatever their origin. A preferred embodiment of the invention includes a combination of a wave plate and electronic tuning to adjust phase delay.